1. Field of the Invention
The present invention relates to an electronic device substrate and its fabrication method, and an electronic device and its fabrication method. Particularly, it relates to an electronic device substrate and its fabrication method, and an electronic device and its fabrication method, which is capable of releasing a core substrate from an electronic device substrate side with weak force, reducing the load of chemical or electrochemical dissolution, or mechanical grinding for exposing a terminal surface to a lower surface, and reducing size.
2. Description of the Related Art
FIGS. 1A and 1B are respectively a front cross-sectional view and a plan view showing an electronic component mount surface of a conventional electronic device. This electronic device 1 comprises a wiring board 2 having through-holes 3, a die pad 4 provided on the wiring board 2, plural wiring patterns 5 each having a metal electrode 5a and an internal connection terminal 5b at both ends and provided on the wiring board 2, an electronic component 6 mounted on the die pad 4 by conductive paste bonding, plural external electrode pads 7 connected to respective lower ends of the through-holes 3 and provided in the lower surface of the wiring board 2, ball-shaped external connection terminals 8 provided respectively for the external electrode pads 7, metallic bonding wires 9 connecting terminals 6a of the electronic component 6 and the internal connection terminal 5b, and a sealing resin 10 provided at the upper surface of the wiring board 2 for covering the electronic component 6 and the bonding wires 9.
The wiring board 2 uses a glass epoxy resin, a polyimide tape, or the like.
The through-holes 3 comprises a conductive body for electrical conduction between the metal electrode 5a and the external electrode pads 7. The die pad 4, wiring pattern 5, and external electrode pads 7 comprise copper foil formed by photochemical etching.
The through-holes 3, die pad 4, wiring pattern 5, and external electrode pads 7 are copper-plated, nickel-base-plated, or gold-plated on an inner surface or on a front surface by electrical or electroless plating.
The electronic device 1 is completed by first mounting the electronic component 6 on the die pad 4 of the wiring board 2, connecting terminals 6a of the electronic component 6 and the internal connection terminal 5b, and sealing with the sealing resin 10 such as an epoxy resin. Typically, finally, spherical external connection terminals 8 such as a solder ball are attached to the external electrode pads 7.
Also, recently, a coreless electronic device that uses no module substrate (hereinafter, a coreless package) has been proposed. For instance, a coreless electronic device is known that die-bonds an electronic component on a base film and wire-bonds between it and a metal base, followed by etching of unwanted portions of the metal base for exposing a terminal and a mount portion (See JP-A-3-94459).
FIG. 2 shows structure of a coreless package, as shown in JP-A-3-94459. This coreless package 20 has the configuration in which the wiring board 2 as an electrical insulative core substrate is removed in FIG. 1, and the backsides of the die pad 4 and wiring pattern 5 are exposed to the package bottom.
Also, as an example of a coreless package with plural electronic components, as shown in JP-A-3-99456, it is known that the plural electronic components and a circuit pattern are connected by wire-bonding, which are integrally sealed with resin, and a protective coating is applied to the bottom of the electronic component package, and the circuit pattern exposed from of an opening of this protective coating is gold-plated for preventing corrosion.
FIGS. 3A-3E show a fabrication method of the coreless package 20 of FIG. 2. First, as in FIG. 3A, there are formed a die pad 4, and plural wiring patterns 5 each having a metal electrode 5a and an internal connection terminal 5b on an electrical insulative transfer film 21 as a core substrate.
Subsequently, as shown in FIG. 3B, an electronic component 6 is mounted on the die pad 4, and as shown in FIG. 3C, this is followed by connecting terminals of the electronic component 6 and the internal connection terminal 5b with bonding wires 9.
Next, as shown in FIG. 3D, the electronic component 6 is sealed with sealing resin 10, followed by removal of the transfer film 21. which results in the coreless package 20, as shown in FIG. 3E. This method transfers a wiring conductor to the sealing resin 10, and is therefore called a transfer method.
As a fabrication method of a coreless package by transferring, a method is known that uses a thick base material instead of a transfer film (see for example, JP-A-9-252014). This method laminates metal foil on the base material, packages and wire-bonds an electronic component on the metal foil, followed by resin sealing and subsequent separation of the resin from the base material.
Further, as an analogous known example of a coreless package transfer method, as shown in JP-A-2002-9196, a fabrication method of a semiconductor device is known that dissolves a metal base positioned in a lower surface. This method fabricates an electronic device by forming a resist pattern in the metal base of a core substrate, forming an opening in a die bonding portion and a portion corresponding to the bonding portion of the resist pattern, and filling the opening with nickel plating, followed by metal plating of its surface, removal of the resist pattern, mounting of an electronic component in the bonding portion, wire-bonding on a gold plating film as the bonding portion, resin sealing thereof, and etching of the metal base.
The conductors such as the die pad, internal connection terminal, wiring pattern, external connection electrode, etc. are formed by photochemical etching of copper foil typically using electrolytic copper foil, rolled copper foil, etc.
FIG. 4 shows structure of a transfer film of FIG. 3. As shown in FIG. 4, this transfer film 21 is provided with an adhesive 22 coated thereover, a die pad 4 and a wiring pattern 5 formed on the adhesive 22, and functional plating 23 applied to the surface of the die pad 4 and the wiring pattern 5.
The functional plating 23 is provided for good connection between a terminal of the electronic component 6 and a metal electrode 5a. This functional plating 23 comprises electroless or electrical nickel plating as base plating, and electroless or electrical gold plating provided thereon.
Typically, the electrical nickel plating is in the thickness range of 0.5-2.0 μm depending on heating conditions in electronic component mounting and wire bonding. Also, the base nickel plating serves as a thermal diffusion prevention film (a barrier film) into the gold plating film of the copper. The gold plating is applied to a surface layer because of high connection reliability of ultrasonic wire-bonding. To enhance wire-bonding, the thicker the better, but the optimal thickness is selected in the range of 0.1-2.0 μm taking account of productivity and cost.
According to the conventional electronic device, in the configuration of FIG. 4, however, there is the problem with very poor adhesion of the gold plating in the functional plating 23 to sealing resin 10. Specifically, when gold plating is applied to the surface of the functional plating 23, the gold does not form a high electronegativity oxide film, which results in poor adhesion to the sealing resin, and degrades reliability of the electronic device.
Also, as shown in FIG. 3, in the fabrication of the coreless package 20, the transfer film 21 is released in a final step, but it is released with the component of the adhesive 22 applied to the transfer film 21 adhering to the backside of the metal electrode 5a, or the transfer film 21 is torn without being completely released.
To avoid this failure, JP-A-2002-9196 describes a method connecting the metal base of the core substrate and the electrode with poor adhesive metals. However, even this method cannot entirely avoid the following:    (1) The first is that because an external electrode portion exposed after releasing the transfer film (core substrate) 21 generally comprises copper or nickel, a terminal portion exposed after releasing must be cleaned with acid, and then coated with electroless gold or tin plating.    (2) The second is that when the bond strength between the transfer film 21 and wiring pattern 5 is stronger than that between the sealing resin 10 and wiring pattern 5, when the transfer film (core substrate) 21 is released, it often slips off the sealing resin 10 with the wiring pattern 5 bonded to the transfer film 21.
To avoid this failure, JP-A-2002-9196 also describes a method thickening the metal of the wiring pattern and fabrication on a periphery an overhanging portion that slightly projects on its sealing resin side, but because the thickness of the wiring pattern is increased, the plating time is long, or the resist film must be removed with a canopy shape held, so that because of these steps, the distance between the adjacent electrodes cannot be reduced, which results in an increase of electronic device dimensions.
On the other hand, in the case of a single layer of the metal of the base of the core substrate, because mechanical durability is required in carrying and fabrication, its thickness needs to be generally 20 μm or more. For that reason, when this is removed by chemical dissolution or mechanical grinding, the processing time is long because of the thickness, which results in a large load of the chemical dissolution or mechanical grinding for exposing a terminal surface from the package backside.
Also, when the metal base is released by applying mechanical stress, the package is curved or cracked by the stress which is an obstacle particularly in forming a thin electronic device.